A transmitting and receiving arrangement which operates using a frequency division duplex method (FDD) can transmit and receive signals simultaneously. The signals are in this case transmitted and received in different frequency bands. Frequency division duplexing methods are used both in analogue communication systems and in mobile radio systems which are based on the CDMA Standard. Standards such as these include, for example, the UMTS, CDMA 2000 and IS 95 Standards.
When signals are transmitted and received simultaneously, crosstalk frequently occurs from the transmission path to the reception path, that is to say a signal to be transmitted is input from the transmission path into the reception path. A transmission signal which has been input into the reception path increases the probability of errors in the demodulation of the received signals, or even completely prevents the reception of signals. This problem becomes worse the higher the level of a signal to be transmitted is in comparison to the level of a simultaneously received signal. If, for example, the mobile station is a very long way away from the base station, the transmission signals must be transmitted from the mobile station to the base station with a relatively high transmission power. At the same time, however, the signals transmitted from the base station are received by the mobile station at only a very low power level. This means that a very high sensitivity must be chosen for the receiver. In this situation, any crosstalk from the transmission path to the reception path is particularly disadvantageous.
In order to suppress interference signals which are caused by crosstalk from the transmission path to the reception path, modern transmitting and receiving arrangements use narrowband surface acoustic wave filters, so-called SAWs. SAW filters are generally arranged both in the transmission path and in the reception path. Narrowband SAW filters have the disadvantage of high production costs as well as an increased surface area requirement and power consumption, particularly for the frequency range that is used for mobile radio.
Instead of complex SAW filters, a control loop arranged between the transmission path and the reception path can be provided in order to attenuate undesirable interference signals. A correction signal is produced as the manipulated variable for the controller, by outputting the transmission signal from the transmission path, and by amplifying and phase-shifting it in a suitable manner. The correction signal produced in this way is superimposed in the detection path on the signal that is present there. If the amplitude and the phase of the correction signal have been chosen appropriately, this procedure leads to the interference signal in the reception path being compensated for by interference with the correction signal. Any control error, that is to say any interference signal occurring after the superimposition of the correction signal in the reception path, is determined by means of a detector arranged in the reception path. The amplitude and the phase of the correction signal are set on the basis of the interference signal component, as determined by the detector, in the reception path.
Ideally, the crosstalk from the transmission signal to the reception path and the correction signal have the same amplitudes and a phase shift of 180°. In this case, the interference signal is completely cancelled out by interference with the correction signal.
The amplitude and the phase of the transmission signal which produces the crosstalk from the transmission path are influenced by a large number of parameters. These parameters include changes in the impedance of the antenna, changes in the gain and phase of the power amplifier arranged in the transmission path, production fluctuations, temperature changes, tolerances in the voltage supply, and frequency changes.
The control loops which have been used for interference signal suppression until now have the disadvantage that it is not possible, or is possible only with very great difficulty, with the detectors used for the control process to distinguish between interference signals which are caused by crosstalk to the reception path and other interference signals in the reception path. This means that the phase and the amplitude of the correction signal can be determined only with difficulty.